To satisfy increasing demands for high-speed packet data, emerging standards for next-generation DS-CDMA (Direct Sequence Code Division Multiple Access) systems are currently extended to cope with higher data rates. Both suggested High Data Rate (HDR) and High Speed Downlink Packet Access (HSDPA) modes consider a time-divided downlink. One key issue for better utilization of scarce radio resources is an appropriate scheduling of users in order to enhance the throughput. Hence, rate control and time-division scheduling algorithms are used in forwarding packet data transmission to utilize the radio resource effectively and support the high transmission rate.
Employing an efficient packet scheduling algorithm is an essential technique in order to improve the total system throughput as well as the peak throughput of each access user. Although always scheduling the user with the highest link quality may maximise capacity, it can result in a performance too unfair among the users. In the RR (Round Robin) method, the packet transmission opportunities are equally assigned to all communicating users within a sector irrespective of the radio link conditions of each user. However, the total system throughput with this RR scheduler becomes much lower than with other scheduling methods. So far, efficient packet scheduling algorithms have been proposed that assign a slot to the access users within a cell based on the radio link conditions which an access user notifies to the base station. A good scheduling algorithm may guarantee the fairness or the QoS of each service while considering the time-varying channel conditions of each user.
Such fairness issues have been studied for many type of systems, not only wireless. For example, in “Asymptotic analysis of proportional fair algorithm” by J. M. Holtzman, Proc. IEEE PIMRIC, vol. 2, pp. 33-37, 2001, the purpose was to schedule the users to get access to the channel the same asymptotical fraction of time but taking advantage of instantaneous channel variations. According to this asymptotical analysis of scheduling, fairness accounts for providing certain channel access time fractions among the users. That is, equal expected throughput is not necessarily guaranteed, rather the access to the channel.
The proportional fair scheduling method assigns transmission packets based on criteria such as a ratio between an instantaneous signal-to-interference power ratio (SIR) and a long-term average SIR value of each user. Another well-known proportional fair scheduling algorithm is the so-called proportional fair throughput (PFT) algorithm which provides a trade-off between throughput maximisation and fairness among users within a cell. In the traditional framework, the PFT algorithm selects the user to be scheduled during the next transmission time interval (TTI) according to a priority metric, which can be expressed as:Pn=Rn/Tnfor a user numbered n, where Rn denotes the throughput which can be offered to user n during the next TTI where this user is scheduled, and Tn denotes the mean or average throughput delivered to this user within a predetermined time period. It is noted that the value Rn is typically time-variant as it depends on the SIR value of this user. The priority metric Pn is calculated for all users sharing the time-multiplexed channel, e.g. the Downlink Shared Channel (DSCH) or the High Speed Downlink Shared Channel (HS-DSCH) as described in the 3 GPP (third generation Partnership Project) specification TS 25.308 V5.4.0.
The user with the largest calculated or determined priority metric is selected to be scheduled during the next TTI. Hence, if the user n has not been scheduled for a long period of time, the monitored average throughput Tn will decrease and consequently cause an increase of the priority Pn of said user.
However, so far, the PFT algorithm does not include any mechanism which helps to guarantee a minimum average throughput to a single user in the system. The same applies to other proportional fair scheduling schemes based on other transmission parameters.